main/srv.c

   1 /*
   2  * Asterisk -- An open source telephony toolkit.
   3  *
   4  * Copyright (C) 1999 - 2005, Digium, Inc.
   5  *
   6  * Mark Spencer <markster@digium.com>
   7  *
   8  * Funding provided by nic.at
   9  *
  10  * See http://www.asterisk.org for more information about
  11  * the Asterisk project. Please do not directly contact
  12  * any of the maintainers of this project for assistance;
  13  * the project provides a web site, mailing lists and IRC
  14  * channels for your use.
  15  *
  16  * This program is free software, distributed under the terms of
  17  * the GNU General Public License Version 2. See the LICENSE file
  18  * at the top of the source tree.
  19  */
  20
  21 /*! \file
  22  *
  23  * \brief DNS SRV Record Lookup Support for Asterisk
  24  *
  25  * \author Mark Spencer <markster@digium.com>
  26  *
  27  * \arg See also \ref AstENUM
  28  *
  29  * \note Funding provided by nic.at
  30  */
  31
  32 #include "asterisk.h"
  33
  34 ASTERISK_FILE_VERSION(__FILE__, "$Revision$")
  35
  36 #include <sys/types.h>
  37 #include <netinet/in.h>
  38 #include <arpa/nameser.h>
  39 #if __APPLE_CC__ >= 1495
  40 #include <arpa/nameser_compat.h>
  41 #endif
  42 #include <resolv.h>
  43 #include <stdio.h>
  44 #include <string.h>
  45 #include <unistd.h>
  46
  47 #include "asterisk/channel.h"
  48 #include "asterisk/logger.h"
  49 #include "asterisk/srv.h"
  50 #include "asterisk/dns.h"
  51 #include "asterisk/options.h"
  52 #include "asterisk/utils.h"
  53 #include "asterisk/linkedlists.h"
  54
  55 #ifdef __APPLE__
  56 #undef T_SRV
  57 #define T_SRV 33
  58 #endif
  59
  60 struct srv_entry {
  61         unsigned short priority;
  62         unsigned short weight;
  63         unsigned short port;
  64         unsigned int weight_sum;
  65         AST_LIST_ENTRY(srv_entry) list;
  66         char host[1];
  67 };
  68
  69 struct srv_context {
  70         unsigned int have_weights:1;
  71         AST_LIST_HEAD_NOLOCK(srv_entries, srv_entry) entries;
  72 };
  73
  74 static int parse_srv(unsigned char *answer, int len, unsigned char *msg, struct srv_entry **result)
  75 {
  76         struct srv {
  77                 unsigned short priority;
  78                 unsigned short weight;
  79                 unsigned short port;
  80         } __attribute__ ((__packed__)) *srv = (struct srv *) answer;
  81
  82         int res = 0;
  83         char repl[256] = "";
  84         struct srv_entry *entry;
  85
  86         if (len < sizeof(*srv))
  87                 return -1;
  88
  89         answer += sizeof(*srv);
  90         len -= sizeof(*srv);
  91
  92         if ((res = dn_expand(msg, answer + len, answer, repl, sizeof(repl) - 1)) <= 0) {
  93                 ast_log(LOG_WARNING, "Failed to expand hostname\n");
  94                 return -1;
  95         }
  96
  97         /* the magic value "." for the target domain means that this service
  98            is *NOT* available at the domain we searched */
  99         if (!strcmp(repl, "."))
 100                 return -1;
 101
 102         if (!(entry = ast_calloc(1, sizeof(*entry) + strlen(repl))))
 103                 return -1;
 104
 105         entry->priority = ntohs(srv->priority);
 106         entry->weight = ntohs(srv->weight);
 107         entry->port = ntohs(srv->port);
 108         strcpy(entry->host, repl);
 109
 110         *result = entry;
 111
 112         return 0;
 113 }
 114
 115 static int srv_callback(void *context, unsigned char *answer, int len, unsigned char *fullanswer)
 116 {
 117         struct srv_context *c = (struct srv_context *) context;
 118         struct srv_entry *entry = NULL;
 119         struct srv_entry *current;
 120
 121         if (parse_srv(answer, len, fullanswer, &entry))
 122                 return -1;
 123
 124         if (entry->weight)
 125                 c->have_weights = 1;
 126
 127         AST_LIST_TRAVERSE_SAFE_BEGIN(&c->entries, current, list) {
 128                 /* insert this entry just before the first existing
 129                    entry with a higher priority */
 130                 if (current->priority <= entry->priority)
 131                         continue;
 132
 133                 AST_LIST_INSERT_BEFORE_CURRENT(&c->entries, entry, list);
 134                 entry = NULL;
 135                 break;
 136         }
 137         AST_LIST_TRAVERSE_SAFE_END;
 138
 139         /* if we didn't find a place to insert the entry before an existing
 140            entry, then just add it to the end */
 141         if (entry)
 142                 AST_LIST_INSERT_TAIL(&c->entries, entry, list);
 143
 144         return 1;
 145 }
 146
 147 /* Do the bizarre SRV record weight-handling algorithm
 148    involving sorting and random number generation...
 149
 150    See RFC 2782 if you want know why this code does this
 151 */
 152 static void process_weights(struct srv_context *context)
 153 {
 154         struct srv_entry *current;
 155         struct srv_entries newlist = AST_LIST_HEAD_NOLOCK_INIT_VALUE;
 156
 157         while (AST_LIST_FIRST(&context->entries)) {
 158                 unsigned int random_weight;
 159                 unsigned int weight_sum;
 160                 unsigned short cur_priority = AST_LIST_FIRST(&context->entries)->priority;
 161                 struct srv_entries temp_list = AST_LIST_HEAD_NOLOCK_INIT_VALUE;
 162                 weight_sum = 0;
 163
 164                 AST_LIST_TRAVERSE_SAFE_BEGIN(&context->entries, current, list) {
 165                         if (current->priority != cur_priority)
 166                                 break;
 167
 168                         AST_LIST_REMOVE_CURRENT(&context->entries, list);
 169                         AST_LIST_INSERT_TAIL(&temp_list, current, list);
 170                 }
 171                 AST_LIST_TRAVERSE_SAFE_END;
 172
 173                 while (AST_LIST_FIRST(&temp_list)) {
 174                         weight_sum = 0;
 175                         AST_LIST_TRAVERSE(&temp_list, current, list)
 176                                 current->weight_sum = weight_sum += current->weight;
 177
 178                         /* if all the remaining entries have weight == 0,
 179                            then just append them to the result list and quit */
 180                         if (weight_sum == 0) {
 181                                 AST_LIST_APPEND_LIST(&newlist, &temp_list, list);
 182                                 break;
 183                         }
 184
 185                         random_weight = 1 + (unsigned int) ((float) weight_sum * (ast_random() / ((float) RAND_MAX + 1.0)));
 186
 187                         AST_LIST_TRAVERSE_SAFE_BEGIN(&temp_list, current, list) {
 188                                 if (current->weight < random_weight)
 189                                         continue;
 190
 191                                 AST_LIST_REMOVE_CURRENT(&temp_list, list);
 192                                 AST_LIST_INSERT_TAIL(&newlist, current, list);
 193                                 break;
 194                         }
 195                         AST_LIST_TRAVERSE_SAFE_END;
 196                 }
 197
 198         }
 199
 200         /* now that the new list has been ordered,
 201            put it in place */
 202
 203         AST_LIST_APPEND_LIST(&context->entries, &newlist, list);
 204 }
 205
 206 int ast_get_srv(struct ast_channel *chan, char *host, int hostlen, int *port, const char *service)
 207 {
 208         struct srv_context context = { .entries = AST_LIST_HEAD_NOLOCK_INIT_VALUE };
 209         struct srv_entry *current;
 210         int ret;
 211
 212         if (chan && ast_autoservice_start(chan) < 0)
 213                 return -1;
 214
 215         ret = ast_search_dns(&context, service, C_IN, T_SRV, srv_callback);
 216
 217         if (context.have_weights)
 218                 process_weights(&context);
 219
 220         if (chan)
 221                 ret |= ast_autoservice_stop(chan);
 222
 223         /* TODO: there could be a "." entry in the returned list of
 224            answers... if so, this requires special handling */
 225
 226         /* the list of entries will be sorted in the proper selection order
 227            already, so we just need the first one (if any) */
 228
 229         if ((ret > 0) && (current = AST_LIST_REMOVE_HEAD(&context.entries, list))) {
 230                 ast_copy_string(host, current->host, hostlen);
 231                 *port = current->port;
 232                 ast_free(current);
 233                 if (option_verbose > 3) {
 234                         ast_verbose(VERBOSE_PREFIX_3 "ast_get_srv: SRV lookup for '%s' mapped to host %s, port %d\n",
 235                                     service, host, *port);
 236                 }
 237         } else {
 238                 host[0] = '\0';
 239                 *port = -1;
 240         }
 241
 242         while ((current = AST_LIST_REMOVE_HEAD(&context.entries, list)))
 243                 ast_free(current);
 244
 245         return ret;
 246 }